miim

## Accuracy Issues

To have accurate measurements during a DCC measurement sequence, the angular table position must remain stationary in between commanded movements. In the more common "non-external" implementation of a rotary table, the machine controller handles reading the table position and when a probe trigger occurs it latches the position of the table as well as the XYZ position and returns all that information. Therefore, PC-DMIS knows the actual table position that corresponded to the XYZ coordinates. If the table tends to drift slightly before the next probe trigger occurs, it doesn’t cause a problem. This is because the machine controller will return the new table position along with the new XYZ coordinates.

With an external rotary table the machine controller is unaware of the presence of a rotary table and therefore no real-time latching of the table position takes place when a probe trigger occurs. In terms of real-time machine controls, there is a substantial delay between when a probe trigger occurs and when PC-DMIS could get a reading of the table position. When the probe trigger occurs, the machine controller must send the XYZ result to PC-DMIS over a communications channel (usually a serial port), PC-DMIS must recognize that event and read the values and would then have to send a position request to the rotary table controller over a different communications channel and wait for its response. Any table drifts during that interval would result in a perceived table position that is not the true position at the time the XYZ positions were recorded. This will cause inaccurate measurement results and there is no way to correct for that.

Due to the above concern, PC-DMIS assumes that the table position remains stable between commanded moves. When PC-DMIS commands the external rotary table to move to a given position it waits for a response before proceeding. The rotary table controller should not respond until the table position is stable and then it should respond with the actual position. This position can vary a little from the requested position with no impact on the accuracy, given the following:

· The controller responds with the actual position

· That position does not change until a new movement request occurs.